If you want to use traditional titles to communicate your roles, Chief Executive Officer is typically focused outside the company and Chief Operating Officer is focused internally. If you want to take a more creative approach to reflect a more modern edge, you could consider calling your partner the Chief Revenue Officer and use the title of Chief Visionary Officer for yourself. You could go even further with titles like Digital Marketing Magician and Sensei, but beware of getting too cute and confusing clients and employees. Remember the most important reason for titles is to clearly communicate position and purpose, and not ego advancement. Happy to share my experience creating and running multiple private and public companies if you'd like to follow up.

The complexity of your situation is similar to multiple acquisitions I have encountered on both sides of the table in private and public transactions. In most situations involving private equity, it is doubtful if the additional cost of a private agent would generate a material gain given your minority position. However, since your situation has the additional complexities of differentiation in EBITDA contribution and negotiation with family members, I would agree with your recommendation to use an external agent. If the cost is a concern, you may want to consider using an advisor/coach/consultant to work on a fixed fee basis instead. I took this approach in selling one of the private companies I ran and was able to complete the transaction at a much lower cost than using an investment banker or agent. I'm happy to share my experience and lessons learned if you would like to follow up with a discussion.